An apparatus for treating aqueous waste material having a biochemical oxygen demand is described in U.S. Pat. application Ser. No. 874 700 filed on Feb. 2, 1978 and comprises a vessel for containing a volume of the material; means for continuously advancing a stream of the aqueous waste material through a passage in communication with a confined stilling zone of the vessel spaced above the bottom of the vessel; means for combining with the said stream a stream of aqueous liquor containing bacterial sludge; baffle means defining said stilling zone and keeping liquor within it separate from an upper layer of clarified liquor which in operation of the apparatus surrounds the stilling zone and from which, in operation of the apparatus, clarified liquid is withdrawn; means for introducing the streams into the stilling zone such that, in operation of the apparatus the momentum of the liquid is substantially reduced before the liquid passes to a lower zone of the vessel in which, in operation of the apparatus, biological treatment of the aqueous material takes place and which contains the bacterial sludge; means for introducing oxygen or oxygen containing gas mixture into the combined stream so as to form discrete bubbles of oxygen gas therein and thereby facilitate dissolution of the oxygen; and means for recycling aqueous material containing bacterial sludge from the liquor in the lower zone and for forming the aforesaid continuously advancing stream of aqueous liquor containing bacterial sludge.
During the development of the above defined apparatus it was noticed that the momentum of the recirculating liquor, particularly in apparatus treating strong effluents with high recirculation rates, persisted and caused difficulties in maintaining a satisfactory clarified liquid zone within the vessel.